Implantable medical devices (IMDs), for example, cardiac pacemakers, defibrillators, neurostimulators and drug pumps, which include electronic circuitry and battery elements, require a housing to contain and hermetically seal these elements within a body of a patient. Many of these IMDs include one or more electrical feedthrough assemblies to provide electrical connection between the elements contained within the housing and components of the IMD external to the housing, for example, sensors and/or electrodes and/or lead wires mounted on an exterior surface, or electrical contacts housed within a connector module, which is mounted on the housing, to provide coupling for lead wires.
A feedthrough assembly for an IMD can be unipolar or multipolar; a unipolar feedthrough assembly includes a single feedthrough member, or pin that extends from an interior to an exterior of the housing through a ferrule, while a multipolar feedthrough assembly includes a plurality of such feedthrough pins extending through a single ferrule. In each type of assembly, the feedthrough pin(s) is/are electrically isolated from the ferrule, and, in the case of the multipolar assembly, from one another, by an insulator element, for example, glass or ceramic, that is mounted within the ferrule and surrounds the feedthrough pin(s). The insulator element is hermetically sealed to the ferrule and to the feedthrough pin(s), typically, by a braze joint.
To reduce the effects of stray electromagnetic interference (EMI) signals that may be collected by lead wires electrically coupled to the feedthrough pins, it is known to incorporate, within feedthrough assemblies, capacitive elements for high frequency filtering. A filtered feedthrough assembly may be formed by mounting the capacitive element within the ferrule after sealing the insulator element to the ferrule and the feedthrough pin(s); the capacitive element typically includes an insulative base, for example, a ceramic monolith, in which electrode plates are embedded, otherwise known as a discoidal-type capacitor. A first set of the electrode plates are electrically coupled to a conductive layer overlaying an inner surface of the capacitor, and a second set of the electrode plates are electrically coupled to another conductive layer overlaying an outer surface of the capacitor.
After mounting the capacitor within the ferrule and around the pin(s), an electrical coupling, or joint, is formed between the first set of electrode plates and the pin(s), and between the second set of electrode plates and the ferrule. This coupling is typically formed by a conductive material extending between the inner surface of the capacitor and each pin, and between the outer surface of the capacitor and the ferrule. If the conductive material is solder, solder pre-forms may be mounted onto an exposed surface of the mounted capacitive element; upon heating the pre-forms, those pre-forms mounted in proximity to each pin flow between the inner surface of the capacitive element and each pin to form the electrical coupling therebetween, and the other pre-form, which is mounted in proximity to the ferrule, flows between the ferrule and the outer surface of the capacitive element to form the electrical coupling therebetween. The use of solder pre-forms can help to improve process consistency and efficiency in manufacturing relatively large quantities of feedthrough assemblies; yet there is still a need for feedthrough assembly features that can further improve consistency and efficiency in manufacturing.